During the production process of various electronic devices such as Flat Panel Displays (FPD) and Printed Circuit Boards (PCB), each device must be thoroughly inspected after various process steps in order to identify and possibly correct production defects and avoid incurring additional costs if the device cannot be repaired. For inspecting such devices, Machine Vision Inspection system Y100 shown in FIG. 1 can be used.
As shown in FIG. 1, a typical Machine Vision Inspection system Y100 for optically inspecting object Y102 uses illumination module Y101 to illuminate the object Y102. The image of the illuminated object Y102 is created using imaging optics Y103 at the location of the camera Y104. The camera Y104 converts the optical image of the object into digital representation, which is automatically processed by the data processing module Y105 in order to, for example, identify various production defects in the object. Two of the major design challenges associated with such inspection systems are:
1. Shaping the illumination light to fit the imaging requirements considering the interaction between the light and the object, which affect the image structure; and
2. How to make the illumination efficient in order to effectively utilize the necessary and available optical power.
As would be appreciated by those of skill in the art, in many situations, the angle of incidence of the illuminating light on the inspected object is very critical to how the image of the object is formed through the imaging optics of the inspection system. For example, while inspecting a reflective object, such as Flat Panel Display (FPD), which contains glass coated with thin film patterned layer, the major portion of the illuminating light reflected by the inspected object is specular (mirror like reflection of light). Creating the image of the object with the specularly reflected light is called Bright Field (BF) imaging.
Dark Field (DF) imaging, which is imaging in scattered light (which excludes the unscattered beam from the image), can also be implemented for inspection purposes. The Dark Field illumination/imaging is very efficient for purposes of inspection, however, the Dark Field illumination and imaging needs to be carefully specified, because it produces a different image type.
With reference to FIG. 2, in the case of non-telecentric imaging optics Y202, the Bright Field illumination beam (Y10) has to be shaped to match the collection angle beam (Y11) over the entire field, as shown in FIG. 2. When Bright Field image of the object is created using such optical configuration, overfilling illumination of the angles like (Y12), results in the light loss and, consequently, in lower performance of the inspection system, see FIG. 3. Thus, in order to ensure that sufficient light gets collected by the imaging sensor to produce an image of acceptable quality, the systems such as one shown in FIGS. 2 and 3 require higher power illuminating light source, which adds additional cost to the price of the system. Moreover, image quality parameters, such as resolution and contrast also suffer.
When the imaging is not telecentric, and the illumination device is a separate module from the imaging optics, it is often difficult to produce a good match between the Bright Field illumination light beam and the imaging optics. Two conventional methods that achieve matching the Bright Field illumination and the imaging are illustrated in FIGS. 4 and 5.
One such option, illustrated in FIG. 4, is tilting the optical axis of the imaging module (Y13), and the projection illumination beam (Y14) with respect to one another. This option has a disadvantage in that it results in defocus at the edges of the field of view (Y15) or (Y16) and makes it difficult to maintain optimal focus over the entire field of view, when it is larger than a thin line.
The second option, illustrated in FIG. 5, is to use a beam splitter (Y17) to direct the illumination light on the object. However, this approach is disadvantageous in that it has low optical power efficiency, and utilizes less than about 25% of the input optical power.
An exemplary implementation of the optical system for Dark Field imaging is shown in FIG. 6. It should be noted that images of inspected objects created using Dark Field imaging are very useful and informative for inspection purposes.
One method for providing Dark Field illumination on the object Y201, is locating the Dark Field illuminating light source (Y18) sufficiently outside the light collection angle area (Y19) of the imaging system. However, in a non-telecentric system, the point (Y20) in the field of view receives the Dark Field illuminating light at a different angle of incidence than the point (Y21) in the field of view. This results in dependence of the illuminating light incidence angle on the object on the position of the point within the field of view. This results, in turn, in change of appearance of the resulting Dark Field image across the field of view. Therefore, the Dark Field imaging requires uniformity in the illuminating light incidence angle across the field, which the system shown in FIG. 6 fails to provide. Illumination uniformity is especially critical in periodical structure inspection applications, where an image of one cell needs to be compared with an image of another. If the cells are differently illuminated, the aforesaid comparison of the respective images could be problematic.
Therefore, there is a need for systems and methods that achieve telecentric illumination and telecentric acquisition of the image of the inspected object in order to provide more uniform illumination, improved performance and better image quality parameters.